Unlike most sensory magnitudes, loudness does not usually adapt--the loudness of a physically steady sound does not decline over time. This research seeks to determine what keeps loudness from adapting and what happens in those special conditions that do lead to adaptation. In this endeavor, we shall explore auditory behavior in response to sounds lasting for many minutes instead of only seconds or milliseconds as in most previous psychoacoustical research. These long-term goals are to be approached by a variety of measurements using primarily the method of successive magnitude estimation which requires the listener to evaluate the loudness of a steady sound over time by assigning a number to it at successively designated intervals, usually every 15 or 20s. Three types of loudness adaptation will be investigated: simple adaptation which occurs only within about 20 dB of absolute threshold, adaptation induced by an intermittent sound in the ear contralateral to the steady sound, and adaptation induced by an intermittent increment in the level of a steady sound. Individual correlations among the three types of adaptation and with other auditory functions including intensity discrimination, temporary threshold shift, and binaural fusion will be detemined in an attempt to uncover the basis for wide individual differences in loundness adaptation. Related to this problem is the finding that teenagers show little induced adaptation, and so youngsters from 10 to 16 years old will be tested. Recovery from induced adaptation, which is slow and unsure, will be further explored. Although, threshold adaptation is often extremely large in retrocochlear impairment, it remains to be shown that such patients experience large amounts of adaptation to a sound well above threshold. This research is not only directly relevant to retrocochlear hearing impairment, but more generally to our conceptualization of the neural coding of loudness which is essential to the understanding and treatment of auditory pathology.